1. Field of the Invention
This invention relates to electrical generating systems, and particularly to a generating system that is capable of setting and producing a constant frequency AC power output from a varying frequency AC input power source, and riding through a short duration power outage.
2. Description of the Prior Art
Equipment which belongs to the same general category as this invention is already on the market and a number of patents exist. Among these are U.S. Pat. No. 4,625,160 by Hucker and U.S. Pat. No. 4,246,531 by Jordan. In the apparatus of Hucker, a variable speed motive power source is coupled mechanically by shaft to a motor and to a generator. The frequency of the generator output is controlled by using signal transformers to transfer signals from a cycloconverter, which is mounted off the shaft, to the rotor windings. In the apparatus of Jordan, a prime mover is coupled by shaft to a generator having an exciter and main unit connected sequentially along the prime mover output shaft. A rotating device exciter is used to generate a frequency difference signal for the generator. Another approach is the system of Ford, U.S. Pat. No. 3,183,431, where the output frequency of the system is maintained constant by changing the frequency of the excitation. The signal is generated by another rotating machine with no static interface. U.S. Pat. No. 4,330,743 by Glennon utilizes a static AC to DC converter which senses motor position. In the apparatus of Gritter, U.S. Pat. No. 4,041,368, a static, non-rotating AC-DC-AC converter is utilized to maintain a constant motor output frequency. U.S. Pat. No. 2,854,617 by Johnson utilizes magnetically coupled devices that vary the field frequency of an alternator and thereby control the output frequency. Alexanderson in U.S. Pat. No. 2,213,945 uses slip rings and external drive circuits to provide signals for controlling AC output frequency. In U.S. Pat. No. 3,764,815, Habock et al. use an externally mounted static frequency converter to control the speed of a gas turbine starter motor.
In the foregoing patents, the control circuit is mounted off and apart from the rotating machine and senses the output voltage and frequency at the load point. Consequently, control of the generator frequency is extremely sensitive to load changes and variations. Furthermore, control of an exciter driving circuit through rotating transformers as typified by the system of Hucker is inefficient and constitutes a weak linking point.
In the foregoing patents, the generator output frequency is fixed and no provision is made for adjusting the output frequency over a given range. If the input AC power should undergo a short time power outage, the described systems will decrease their speed of rotation and the output frequency will drop significantly. Thus, there exists a need for an improved constant speed and frequency system that is not sensitive to load changes and variations; whose output frequency can be selected and adjusted over a given range, and which can ride-through a short time power outage while maintaining the output frequency constant.